Abstract: Consistent with a child × environment model it was hypothesized that anxious solitude and the middle school transition interact to influence peer mistreatment over time. A sample of 688 children participated in peer- and self-reported behavioral nominations assessing exclusion and victimization twice yearly from the fall of third grade through the spring of seventh grade, including the transition to middle school in the fall of sixth grade. Classroom emotional support was observed yearly. Piecewise growth curve models were used to model the level and slope of peer mistreatment outcomes before, at, and after the middle school transition, and assess child-driven, environment-driven, and child × environment effects. Observed classroom emotional support decreased at the middle school transition. According to peer-reports, high vs. average anxious solitary children experienced greater relative improvements in exclusion and victimization at the transition (child × environment effects). However, in both elementary and middle school, elevated anxious solitude predicted elevated peer exclusion and victimization (child-driven effects). Consistent with environment-driven effects, peer- and self-reports indicated decreased exclusion at the transition, indicating that exclusion occurred less frequently when peer-groups were not well-established. Furthermore, peer-, but not self-, reports indicated decreased victimization at the middle school transition, although overall both reporters indicated lower levels of victimization in middle vs. elementary school. Because improvements in peer mistreatment did not correspond to increased classroom emotional support, rearrangement of peer social structure appeared to have the biggest impact on peer mistreatment after the middle school transition.